For the sake of convenience of explanation but not for limitation, description will particularly refer to an ethylene-propylene-non-conjugated diene terpolymer (hereinafter abbreviated as EPDM) as a typical example of EPR.
EPDM is sulfur-vulcanizable but has no unsaturated bond in the main chain thereof and is therefore excellent in ozone resistance, thermal aging resistance, weather resistance, and the like. It is also economically advantageous in that inorganic fillers, e.g., calcium carbonate and clay, can be compounded with in larger amounts than with other weather-resistant rubber materials, e.g., chloroprene rubber (CR), IIR, CSM. NBR/PVC.
Hence, EPDM rubber compositions have been widely used as molding materials for rubber articles in various industrial fields. For example, they have been chief molding materials for producing automobile parts, e.g., weatherstrips.
However, when large amounts of softeners are added to EPDM compositions having high contents of inorganic fillers for the purpose of improving processability, the resulting EPDM compositions tend to lose balance with the softeners due to non-polarity of EPDM. Rubber articles obtained from such EPDM compositions are therefore liable to suffer from bleeding of the softeners, which impairs appearance and feel of the articles.
As a counter measure against bleeding, attempts have been made to alter the kinds of compounding additives including softeners or to reduce amounts of compounding additives to be added but turned out to involve many problems of processability, cost, etc. It has thus been difficult to completely prevent bleeding in EPDM rubber articles.
The terminology "softeners" as used herein includes petroleum process oils, such as paraffin oils, naphthene oils, and aromatic oils, vegetable fats and oils, and plasticizers.
The terminology "bleeding" as used herein means a phenomenon that compounding additives such as softeners ooze out from the inside to the surface of rubber articles.
In order to prevent blooming (a phenomenon that compounding additives in rubber articles bloom to the surface and crystallize) of sulfur, it has been proposed to compound sulfur and sepiolite powder with a diene type rubber composition to thereby make sulfur be adsorbed as insoluble sulfur to the inside of sepiolite crystals as disclosed in JP-A-59-126442 (the term "JP-A" as used herein means an "unexamined published Japanese patent application"). This technique is to utilize sepiolite as a supporting carrier for insoluble sulfur and is therefore irrelevant to the present invention aiming at prevention of bleeding, giving no influence on patentability of the present invention.